WO2025233023A1

WO2025233023A1 - Polarization maintaining photonic crystal fibre

Info

Publication number: WO2025233023A1
Application number: PCT/EP2025/056400
Authority: WO
Inventors: Jens Kristian LYNGSØE; Martin Dybendal Maack
Original assignee: NKT Photonics AS
Current assignee: NKT Photonics AS
Priority date: 2024-05-06
Filing date: 2025-03-10
Publication date: 2025-11-13
Anticipated expiration: 2026-11-06

Abstract

A polarization maintaining, PM, photonic crystal fibre, PCF, (100) comprising a core region (102), inner cladding region (104) and outer cladding region (110). The inner cladding region comprises microstructure elements (106) extending in a cladding background material (108) having a first coefficient of thermal expansion. The outer cladding region (110) comprises cladding background material (112), a first stress applying region, SAR, (114) and a second SAR (116) each extending through the cladding background material (112) of the outer cladding region. The SARs each comprise stress elements (118) of a material having a second coefficient of thermal expansion The stress elements of each SAR are arranged such that each stress element is connected to a number in the range one to five other stress elements. The locations of the first SAR and the second SAR relative to the core region and the microstructure elements, and the first and second coefficients of thermal expansion are configured to cause stress induced birefringence in the core region. A preform for fabricating the PM PCF 100 is also provided.

Description

POLARIZATION MAINTAINING PHOTONIC CRYSTAL FIBRE

Technical Field

The invention relates to a polarization maintaining, PM, photonic crystal fibre, PCF. The invention further relates to a preform for fabricating the PM PCF.

Background

Large mode area, LMA, polarization maintaining, PM, photonic crystal fibres, PCF, (also known as ‘micro-structured fibres’) can be designed to be endlessly single-mode (i.e. to have no higher order mode cut-off), deliver excellent mode quality at all wavelengths, and exhibit low loss while keeping an almost constant mode field diameter. This makes them ideal for multi-wavelength transmission and delivery of short optical pulses, and to be used as single-mode PM fibre pigtails. Each of these applications of LMA PM PCFs require, or benefit from, the LMA PM PCF being connectorized, i.e. having an optical fibre connector provided at one end. The LMA PM PCF can thus be turned into an optical fibre cable or patch cord that can be easily connected to an optical input and/or output of an optical device.

WO 2005/059612 describes a LMA PM PCF comprising stress rods provided within the PCF microstructure cladding region, which cause stress induced birefringence in the PCF core region.

Summary

It is an object to provide an improved polarization maintaining photonic crystal fibre, PM PCF.

It is a further object to enable improved cladding mode suppression in a PM PCF.

An aspect of the disclosure provides a polarization maintaining, PM, photonic crystal fibre, PCF. The PM PCF comprises a core region for propagating light in a longitudinal direction, an inner cladding region surrounding the core region and an outer cladding region surrounding the inner cladding region. The inner cladding region comprises a plurality of microstructure elements extending in the longitudinal direction in a cladding background material having a first coefficient of thermal expansion. The outer cladding region comprises a cladding background material having the first coefficient of thermal expansion, a first stress applying region and a second stress applying region. The first stress applying region and the second stress applying region each extend in the longitudinal direction through the cladding background material of the outer cladding region. The first stress applying region and the second stress applying region each comprises a plurality of stress elements of a material having a second coefficient of thermal expansion different to the first coefficient of thermal expansion. The stress elements of each stress applying region extend in the longitudinal direction and are arranged such that each stress element is connected to a number in the range one to five of other said stress elements. The locations of the first stress applying region and the second stress applying region relative to the core region and the microstructure elements, the first coefficient of thermal expansion, and the second coefficient of thermal expansion are configured to cause a stress induced birefringence in the core region of the PM PCF.

The structure of the PM PCF advantageously enables improved suppression of unwanted cladding modes of light propagating in the PM PCF. The arrangement of the stress elements in each stress applying region avoids the creation of 7-element clusters of stress elements, which advantageously mitigates formation of localized cladding modes within the stress applying regions.

In certain embodiments, the stress elements of each stress applying region are arranged such that there are no areas of the cladding background material that are enclosed by stress elements. This ensures that regions of cladding background material within stress applying regions are connected to the cladding background material of the outer cladding region, advantageously mitigates formation of localized cladding modes within the stress applying regions and may enable improved suppression of unwanted cladding modes of light propagating in the PM PCF. The stress elements may be symmetrically arranged outside the inner cladding region, such that the two stress applying regions are located in a horizontal plane intersecting the core region. The horizontal plane may be parallel to a side of a hexagon formed by the inner cladding region. Preferably, the stress elements in the stress applying regions are arranged adjacent to the inner cladding region such that none of said stress elements are located in the aforementioned layers. In other words, the stress applying regions are not overlapping with the inner cladding region.

In certain embodiments, each stress applying region may comprise at least 10 stress elements, such as 20 stress elements, such as at least 28 stress elements, such as at least 35 stress elements.

Advantageously, increasing the number of stress elements will provide a larger birefringence in the PM PCF.

In certain embodiments each stress applying region may comprise at least 10 stress elements and each stress element has less than six neighbouring stress elements, such that 7-element clusters of stress elements are avoided within each stress applying region.

In certain embodiments, the stress applying regions do not overlap the inner cladding region.

In certain embodiments, the microstructure elements are arranged in a first lattice. The stress elements of each said stress applying region are located at certain lattice points of a respective second lattice such that not every lattice point has a stress element. This avoids the creation of 7-element clusters of stress elements, which advantageously mitigates formation of localized cladding modes within the stress applying regions. In certain embodiments, lattice points of the second lattice without stress elements are filled by the cladding background material of the outer cladding region. This ensures that regions of cladding background material within stress applying regions are connected to the cladding background material of the outer cladding region, advantageously mitigates formation of localized cladding modes within the stress applying regions and may enable improved suppression of unwanted cladding modes of light propagating in the PM PCF.

In certain embodiments, lattice points of the second lattice without stress elements are not surrounded by stress elements. This avoids the creation of 7-element clusters of stress elements, which advantageously mitigates formation of localized cladding modes within the stress applying regions.

In certain embodiments, the first lattice and the second lattice have the same lattice structure. This may advantageously enable improved shape and/or positioning of the stress applying regions within the outer cladding region, which may advantageously improve the cladding mode suppression.

In an embodiment, the first lattice and the second lattice are hexagonal lattices, i.e., where the microstructural elements and the plurality of stress elements are arranged in hexagonal lattices.

In certain embodiments, microstructure elements are arranged with a first pitch between centres of neighbouring microstructure elements and stress elements are arranged with a second pitch between centres of neighbouring stress elements.

In certain embodiments, the core region is configured to propagate light at a specified operating wavelength. A ratio of the second pitch to the operating wavelength is greater than 12.

In certain embodiments, the ratio of the second pitch to the operating wavelength is greater than 13. In certain embodiments, the ratio of the second pitch to the operating wavelength is greater than 14. In certain embodiments, the ratio of the second pitch to the operating wavelength is greater than 15. In certain embodiments, the ratio of the second pitch to the operating wavelength is up to 25.

In certain embodiments, the first pitch is the same as the second pitch. Using the same pitch may provide the advantage of easier stacking of the preform from which the PM PCF is produced.

In certain embodiments, the operating wavelength is lower than 800 nm. In certain embodiments, the operating wavelength is lower than 750 nm. In certain embodiments, the operating wavelength is lower than 700 nm. In certain embodiments, the operating wavelength is lower than 650 nm. In certain embodiments, the operating wavelength is down to 400 nm. The structure of the PM PCF advantageously enables improved cladding mode suppression at relatively short wavelengths. In certain embodiments, the second pitch is at least 5 |j.m. In certain embodiments, the second pitch is at least 10 |j.m. In certain embodiments, the second pitch is up to 20 |j.m. The structure of the PM PCF advantageously enables improved cladding mode suppression in larger core PM PCFs.

In certain embodiments, the core region has a diameter of at least 5 |j.m. In certain embodiments, the core region has a diameter of at least 8 |j.m. In certain embodiments, the core region has a diameter of at least 10 |j.m. In certain embodiments, the core region has a diameter of at least 15 |j.m. In certain embodiments, the core region has a diameter of up to 30 |j.m. The structure of the PM PCF advantageously enables improved cladding mode suppression in larger core PM PCFs.

The core region is preferably solid. In an embodiment, the core region comprises a silica material.

In certain embodiments, the stress elements comprise rods of a first material having a first refractive index. Each stress applying region additionally comprises a second material forming nodes and bridges between the rods. The second material has a second refractive index higher than the first refractive index. The arrangement of the stress elements within the stress applying regions advantageously reduces the number of cladding modes that are supported by the nodes and bridges. This advantageously mitigates formation of localized cladding modes within the stress applying regions and may enable improved suppression of unwanted cladding modes of light propagating in the PM PCF.

In certain embodiments, the first material is Boron-doped Silica and the second material is Silica, thereby taking advantage of industry standard, proven technology for manufacturing optical waveguides. Other materials may be used though, e.g. fluoride (e.g. fluorozirconate), tellurite, phosphate or chalcogenide based glasses or polymer materials, whereby the fibre may be optimised for particularly strong birefringence and/or specific wavelengths/wavelength ranges.

In certain embodiments both the first and the second material are solid materials, such that the stress elements are formed of solid rods.

In certain embodiments, the first stress applying region and the second stress applying region are located in the cladding background material outside the inner cladding region.

In certain embodiments the stress elements are arranged in a two-dimensional array.

In certain embodiments, the stress elements are arranged in two or more pairs of rows, wherein the pairs of rows are at least partially separated by a section of cladding background material.

In certain embodiments, the microstructure elements in the inner cladding region form a hexagonal arrangement such that the inner cladding region has a substantially hexagonal cross-sectional shape with a 6-fold symmetry. In an embodiment, the sides of the inner cladding region facing the stress applying regions are straight or convex.

In certain embodiments, a first side of the first stress applying region is aligned with a first side of the inner cladding region and a first side of the second stress applying region is aligned with a second side of the inner cladding region, opposite the first side of the inner cladding region. This may enable improved shape and/or position of the stress applying regions within the outer cladding region, which may advantageously provide increased birefringence in the fibre core region and improved cross-sectional roundness of the PM PCF. A PM PCF having this structure may advantageously be drawn without tilt of the stress applying regions occurring during the fibre draw.

In certain embodiments, the first stress applying region and the second stress applying region respectively define a first recess and a second recess in cross-section, and wherein the first recess is aligned with a first corner of the inner cladding region and the second recess is aligned with a second corner of the inner cladding region, opposite the first corner. This may enable improved shape and/or position of the stress applying regions within the outer cladding region, which may advantageously provide that the stress applying regions can provide the strongest effect and thereby increased birefringence in the fibre core region and improved cross-sectional roundness of the PM PCF even for relatively small outer diameters of the PM PCF. A PM PCF having this structure may advantageously be drawn without tilt of the stress applying regions occurring during the fibre draw.

In certain embodiments, the hexagonal arrangement in the inner cladding region comprises 4 layers of microstructure elements surrounding the core region, such as 5 layers of microstructure elements surrounding the core region.

The 4 layers of microstructure elements may comprise at least 60 microstructure elements. In other embodiments, the inner cladding region comprises at least two layers of microstructure elements surrounding the core region, or at least three layers of microstructure elements surrounding the core region.

The light guiding properties of the PM PCF generally improves with increasing number of layers of microstructure elements, while the stress induced birefringence decreases with increasing distance between the core region and the stress applying regions. Furthermore, the birefringence increases by increasing the number of stress elements, up to a certain number of stress elements for which the stress elements will cross the 45° angle between the x-axis and y-axis of the fibre. When the stress elements cross the said angle, birefringence will be induced in the opposite axis of the fibre, thus leading to a decrease of birefringence. For some values of the first pitch a strong guidance and a strong birefringence is obtained with 4 layers of microstructure elements. In case of 4 layers of microstructure elements, this corresponds to 60 microstructure elements as shown in figures 1-13. The layers of microstructure elements may be substantially complete to improve the guiding properties of the core region.

In an embodiment, 4 or 5 layers of microstructure elements is arranged between the core region and the outer cladding region along an axis extending between the stress applying regions.

In certain embodiments, the cladding background material is Silica and the stress elements are formed of Boron-doped Silica, thereby taking advantage of industry standard, proven technology for manufacturing optical waveguides. Other materials may be used though, e.g. fluoride (e.g. fluorozirconate), tellurite, phosphate or chalcogenide based glasses or polymer materials, whereby the fibre may be optimised for particularly strong birefringence and/or specific wavelengths/wavelength ranges.

Corresponding embodiments and advantages apply to the preform described below.

Another aspect of the present disclosure provides a preform for fabricating the PM PCF. The preform comprises stacked longitudinal preform elements comprising a preform core rod, a plurality of preform inner cladding capillary tubes of the cladding background material, a plurality of preform stress element rods of the material having the second coefficient of thermal expansion, a plurality of preform outer cladding rods of the cladding background material and a preform stacking tube. The preform inner cladding capillary tubes are stacked in an inner cladding region around the preform core rod. The preform stress element rods are arranged in a first stress applying region and in a second stress applying region. Within each of the first stress applying region and the second stress applying region the preform stress element rods are stacked such that each preform stress element rod is in contact with a number in the range one to five of other said preform stress element rods. The outer cladding rods are stacked in an outer cladding region around the inner cladding region, the first stress applying region and the second stress applying region. The preform core rod, the preform inner cladding capillary tubes, the preform stress element rods and the preform outer cladding rods are located within the preform stacking tube.

In certain embodiments, the preform stress element rods comprise rods of a first material having an outer shell layer of a second material. The first material has a first refractive index and the second material has a second refractive index higher than the first refractive index.

In certain embodiments, the first material is Boron-doped Silica and the second material is Silica.

In certain embodiments, preform stress element rods have an outer diameter, OD, and the outer shell layers have an inner diameter, ID. A ratio ID/OD of the ID to the OD is in the range 0.8 to 0.95.

In certain embodiments, the preform stress element rods are arranged outside the inner cladding region in the first stress applying region and in the second stress applying region. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the drawings

Figures 1 , 6, 8, 10 and 12 are schematic drawings of cross sections of embodiments of a PM PCF;

Figure 2 shows a photomicrograph of a cross section of an embodiment of a PM PCF;

Figure 3 shows a photomicrograph of the PM PCF of Figure 2 with light transmission in the PM PCF;

Figure 4 shows a photomicrograph of a cross section of a PM PCF in which each stress element is connected to up to six other stress elements, with light transmission in that PM PCF; and

Figures 5, 7, 9, 11 and 13 are schematic drawings of cross sections of embodiments of a preform for fabricating the PM PCFs.

Detailed description

The same reference numbers are used for corresponding features in different embodiments.

Optical fibres according to the present invention (termed photonic crystal fibres) have a longitudinal direction and a cross section perpendicular thereto. The cross section of a photonic crystal fibre may vary along its length but is typically constant.

Referring to Figure 1 , an embodiment provides a polarization maintaining, PM, photonic crystal fibre, PCF, 100 comprising a core region 102 for propagating light in a longitudinal direction of the PM PCF, an inner cladding region 104 surrounding the core region, and an outer cladding region 1 10 surrounding the inner cladding region.

The inner cladding region comprises a plurality of microstructure elements in the form of microstructural holes 106 provided in cladding background material 108. The microstructural holes extend in the longitudinal direction of the PM PCF and are arranged (as viewed in the cross-section of the PM PCF 100, as illustrated in Figure 1) in a first lattice. The first lattice is a substantially hexagonal lattice (i.e. the lengths of the edges of the unit cell of the lattice structure, and the angles between them, result in unit cells that are substantially hexagonal in shape) and has a first pitch between centres of adjacent microstructural holes (i.e. between adjacent unit cells). The inner cladding region may comprise at least 30 microstructural holes, such as at least 50 microstructural holes, such as at least 60 microstructural holes.. The cladding background material has a first coefficient of thermal expansion, and may, for example, be Silica. The core region may also be Silica.

The outer cladding region 110 comprises cladding background material 112, a first stress applying region 114 and a second stress applying region 116. The cladding background material 112 has the first coefficient of thermal expansion, i.e. it is the same material as the cladding background material of the inner cladding region. The cladding background material 112 may, for example, be Silica. The stress applying regions do not overlap the inner cladding region.

The first stress applying region and the second stress applying region each extend in the longitudinal direction of the PM PCF through the cladding background material 1 12 of the outer cladding region 110. The first stress applying region and the second stress applying region are located outside the inner cladding region.

The inner cladding region 104 has a substantially hexagonal cross-sectional shape. The first stress applying region 114 defines a first recess 122 and the second stress applying region 116 defines a second recess 124, as viewed in cross-section (as illustrated in Figure 1). The first stress applying region is located with a first corner of the inner cladding region aligned with the first recess. The second stress applying region is located with a second corner of the inner cladding region, opposite the first corner, aligned with the second recess.

The first stress applying region and the second stress applying region are each formed of a plurality of stress elements 118. The stress elements are formed of a material having a second coefficient of thermal expansion different to the first coefficient of thermal expansion. The stress elements may, for example, comprise rods of Boron-doped Silica.

The stress elements 118 of each stress applying region extend in the longitudinal direction of the PM PCF 100. Within each stress applying region 114, 116, the stress elements are arranged such that each stress element is connected to a number in the range one to five other stress elements. As can be seen in Figure 1 , the stress elements 118 in each stress applying region 114, 116 are arranged in two pairs of chevron-like rows, with a section of cladding background material 112 at lattice points 120 between the pairs of rows.

Each stress applying region may comprise at least 10 stress elements, such as at least 20 stress elements, such as at least 28 stress elements, such as at least 35 stress elements.

The locations of the first stress applying region 1 14 and the second stress applying region 116 relative to the core region 102 and the microstructure elements 106, the first coefficient of thermal expansion, and the second coefficient of thermal expansion are configured to cause a stress induced birefringence in the core region 102 of the PM PCF 100.

Anisotropic deformation of the core region due to the stress applying regions or a resulting anisotropic strain field (or stress field) induced in the core region by the stress applying regions results - due to the elasto-optic effect - in the material of the core region becoming birefringent, whereby different polarization states experience different refractive indices, providing a means for separating the two polarization states of a given longitudinal mode, and optionally eliminating one of them. An ‘anisotropic strain field’ will be understood to mean a strain field that is different in size in two different directions of a cross section of the core region. The combination of a micro-structured fibre (enabling single-mode operation over a large wavelength range) and the induction of an anisotropic strain field in the core region (providing birefringence) enable single mode operation of the waveguide with substantially constant birefringence over a large wavelength range.

In the present context, the ‘core region’ is defined - when viewed in a cross section perpendicular to a longitudinal direction of the fibre - as a (typically central) light-propagating part of the fibre. The core region is limited in a radial direction by the microstructure elements of the inner cladding region.

By the ‘coefficient of thermal expansion’ is generally meant the volume coefficient of thermal expansion.

The difference between the first coefficient of thermal expansion and the second coefficient of thermal expansion is sufficiently large to generate a total strain in the core region to result in a birefringence of at least 0.5 x 10^-5, such as larger than 5 x 10^-5, such as larger than 10^-4. The degree of birefringence of an optical waveguide is defined by the difference between the effective mode indices in the two primary polarisation states.

Configuring the first coefficient of thermal expansion and the second coefficient of thermal expansion to cause a stress induced birefringence in the core region of the PM PCF can include configuring the relative magnitude of the coefficients of thermal expansion, depending on the materials used and their temperature dependencies.

In an embodiment, the relative difference between the first coefficient of thermal expansion and the second coefficient of thermal expansion numerically is larger than 1 %, such as larger than 5%, such as larger than 10%. In an embodiment, the numerical value of the difference between the first coefficient of thermal expansion and the second coefficient of thermal expansion is larger than 0.1 *10^-6 K^-1, such as larger than 0.5*10^-6 K^-1, such as larger than 1.0*10^-6 K^-1.

Other arrangements of the stress elements, in which each stress element is connected to a number in the range one to five other stress elements, are possible, some of which are described below.

In certain embodiment, the stress elements are arranged so that that there are no areas of the cladding background material 112 that are enclosed by stress elements.

In certain embodiments, the core region 102 is configured to propagate light at a specified operating wavelength. The operating wavelength may be lower than 800 nm, such as lower than 750 nm, such as lower than 700 nm, such as lower than 650 nm, such as down to 400nm. In certain embodiments, the microstructural holes 106 are arranged with a first pitch between centres of neighbouring microstructure elements and the stress elements 118 are arranged with a second pitch between centres of neighbouring stress elements.

In certain embodiments, a ratio of the second pitch to the operating wavelength is greater than 12. The ratio of the second pitch to the operating wavelength may be greater than 13, such as greater than 14, such as greater than 15, such as up to 25.

In certain embodiment, the second pitch is at least 5 |j.m, such as at least 10 |j.m, such as up to 20 |j.m.

For example, the core region 102 may have a diameter of 15 |j.m and be configured to propagate light at an operating wavelength lower than 650 nm, such as 532 nm, and the stress elements are arranged with a second pitch of 10 |j.m. This gives a ratio of the second pitch to operating wavelength of 18.8.

In another example, the core region 102 may have a diameter of 10 |j.m and be configured to propagate light at an operating wavelength of 400 nm, and the stress elements are arranged with a second pitch of less than 7 |j.m, such as 6.5 |j.m. This gives a ratio of the second pitch to operating wavelength of 16.25.

In another example, the core region 102 may have a diameter of 15 |j.m and be configured to propagate light at an operating wavelength of 400 nm and the stress elements are arranged with a second pitch of 10 |j.m. This gives a ratio of the second pitch to operating wavelength of 25.

In another example, the core region 102 may have a diameter of 25 |j.m and be configured to propagate light at an operating wavelength of 1064 nm and the stress elements are arranged with a second pitch of 16.5 |j.m. This gives a ratio of the second pitch to operating wavelength of 15.66.

In certain embodiments, the first pitch is the same as the second pitch.

In certain embodiments, as illustrated in the expanded section in Figure 1 , the stress elements 118 of each stress applying region are located at certain lattice points of a respective second lattice such that not every lattice point has a stress element. The second lattice is a substantially hexagonal lattice (i.e. the lattice structure is formed of lattice cells that are substantially hexagonal in shape) and has the same lattice structure and pitch as the first lattice, i.e. the second lattice has the same pitch between neighbouring, i.e. adjacent, lattice points as is between the centres of neighbouring microstructure elements of the inner cladding region. Within each stress applying region 114, 116, the lattice points 120 without stress elements are filled by the cladding background material 112 of the outer cladding region and are not surrounded (i.e. in contact on all sides) by stress elements. Each stress applying region 114, 116 additionally comprises nodes 126 and bridges 128 between the rods, formed of a material having a refractive index higher than the material of the stress elements. In this embodiment, the nodes and bridges are formed of Silica.

Referring to Figure 2, an embodiment provides a polarization maintaining, PM, photonic crystal fibre, PCF, 150 comprising a core region 102 for propagating light in a longitudinal direction of the PM PCF, an inner cladding region 104 surrounding the core region, and an outer cladding region 110 surrounding the inner cladding region, as described above with reference to Figure 1.

The core region 102 has a diameter of 15 |j.m and is formed of Silica. The core region 102 is configured to propagate light at 532 nm.

The cladding background material 108, 112 is Silica. The stress elements 118 are rods of Boron-doped Silica and the nodes and bridges between the stress elements are formed of Silica. The microstructural holes 106 are formed in a hexagonal lattice having a first pitch of 10 |j.m between the centres of neighbouring holes. The inner cladding region may comprise at least 30 microstructural holes, such as at least 50 microstructural holes, such as at least 60 microstructural holes.

The stress element rods are provided at lattice points of a hexagonal lattice having a second pitch of 10 |j.m, i.e. a pitch of 10 |j.m between the centres of neighbouring stress element rods and a pitch of 10 |j.m between the centre of a lattice point 120 filled with cladding background material and neighbouring stress element rods. This arrangement of the stress element rods 118 avoids 7-element clusters of rods by removing 7 elements from each stress applying region 114, 116, and also ensures that the lattice points 120 within the stress applying regions that are filled with cladding background material are connected to the outer cladding region 110 cladding background material.

Figure 3 shows a photomicrograph of the PM PCF 150 of Figure 2 with light transmission in the PM PCF. It can be clearly seen that light is strongly guided in the core region 102 with very little light showing in the nodes and bridges of the stress applying regions 114, 116. This indicates that few, if any, localized cladding modes of light are formed within the stress applying regions, meaning there is good suppression of unwanted cladding modes of light propagating in the PM PCF 150.

By way of comparison, Figure 4 shows a photomicrograph of a PM PCF 160 comprising a core region 162, an inner cladding region 164 comprising microstructural holes 166, an outer cladding region 168, a first stress applying region 170 and a second stress applying region 172 formed of stress rods 174. The parameters of the PM PCF 160 are all the same as for the PM PCF 150 but the stress applying regions have a different structure. The stress rods 174 are connected to up to six other stress rods, meaning that the stress applying regions 170, 172 of this PM PCF 160 include 7-element clusters of rods, i.e. one rod surrounded by six others. It can be clearly seen in Figure 4 that while light is still guided in the core 162, there is a significant amount of light propagating in localized cladding modes within the bridges and nodes between the stress rods, i.e. there is less suppression of unwanted cladding modes in the PM PCF 160.

As the second pitch increases, localized cladding modes are better confined in the Silica nodes and bridges. Usually, the localized cladding modes in the stress applying regions are not a problem in PM PCFs with core sizes of 5 |j.m and 10 |j.m, but for a core size of 15 |j.m they are often guided up to around 600 nm. Reducing the thickness of the Silica nodes and bridges, by reducing the thickness of the Silica shell on the preform stress element rods, as described below, is one way to push the modes out of the stress applying regions. But even using commercial off the shelf preform stress element rods with the thinnest possible Silica shell, for example Prysmian® Draka Boron doped stress rods with Silica shells having the highest inner diameter/outer diameter ratio available, localized cladding modes are still present. The structure of the PM PCFs 100, 150 of the present disclosure advantageously mitigates formation of localized cladding modes within the stress applying regions in a different way, namely by avoiding the creation of 7-element clusters of stress elements.

Referring to Figure 5, an embodiment provides a preform 200 for fabricating the PM PCFs 100, 150 of Figures 1 and 2. The preform comprises stacked longitudinal preform elements comprising a preform core rod 202, a plurality of preform inner cladding capillary tubes 204 of the cladding background material, a plurality of preform stress element rods 206 of the material having the second coefficient of thermal expansion, a plurality of preform outer cladding rods 208 of the cladding background material and a preform stacking tube 210.

The preform inner cladding capillary tubes 204 are stacked in an inner cladding region 212 around the preform core rod. The preform stress element rods 206 are arranged in a first stress applying region 222 and in a second stress applying region 224. Within each stress applying region 222, 224 the preform stress element rods are stacked in two pairs of chevron-like rows, with a chevron-like row of preform outer cladding rods 208 between the pairs of rows.

Other arrangements of the preform stress element rods, in which each preform stress element rod is in contact with a number in the range one to five other preform stress element rods, are possible, some of which are described below.

The preform outer cladding rods 208 are stacked in an outer cladding region 220 around the inner cladding region, the first stress applying region and the second stress applying region. The preform core rod, the preform inner cladding capillary tubes, the preform stress element rods and the preform outer cladding rods are located within the preform stacking tube.

In certain embodiments, the preform stress element rods 206 comprise rods of a first material having an outer shell layer of a second material. The first material has a first refractive index and the second material has a second refractive index, higher than the first refractive index. For example, the preform stress element rods 206 may comprises Boron-doped Silica rods having a Silica shell.

In certain embodiments, the preform stress element rods 206 have an outer diameter, OD, and the outer shell layers have an inner diameter, ID. A ratio ID/OD of the ID to the OD is in the range 0.8 to 0.95.

In certain embodiment, the preform stress element rods 206 are arranged so that that there are no preform outer cladding rods 208 that are surrounded by preform stress element rods.

Referring to Figure 6, an embodiment provides a polarization maintaining, PM, photonic crystal fibre, PCF, 300 comprising a core region 102 for propagating light in a longitudinal direction of the PM PCF, an inner cladding region 104 surrounding the core region, and an outer cladding region 310 surrounding the inner cladding region.

The core region and the inner cladding region are as described above with reference to Figure 1.

The outer cladding region 310 comprises cladding background material 112, a first stress applying region 314 and a second stress applying region 316. The cladding background material 112 has the first coefficient of thermal expansion, and may, for example, be Silica.

The first stress applying region and the second stress applying region each extend in the longitudinal direction of the PM PCF through the cladding background material 112 of the outer cladding region 310. The first stress applying region and the second stress applying region are located outside the inner cladding region. The first stress applying region and the second stress applying region are each formed of a plurality of stress elements 118, as described above. The stress applying regions do not overlap the inner cladding region.

The first stress applying region 314 defines a first recess 122 and the second stress applying region 316 defines a second recess 124, as described above.

Within each stress applying region 314, 316, the stress elements are arranged such that each stress element is connected to a number in the range one to five other stress elements. As can be seen in Figure 6, the stress elements 118 in each stress applying region 314, 316 are arranged so that a section of cladding background material 112 comprising three branches of three lattice points 320 is formed with each stress applying region and connected to the cladding background material of the outer cladding region 310.

The locations of the first stress applying region 314 and the second stress applying region 316 relative to the core region 102 and the microstructure elements 106, the first coefficient of thermal expansion, and the second coefficient of thermal expansion are configured to cause a stress induced birefringence in the core region 102 of the PM PCF 300.

Referring to Figure 7, an embodiment provides a preform 400 for fabricating the PM PCF 300 of Figure 6. As described above with reference to Figure 5, the preform 400 comprises stacked longitudinal preform elements comprising a preform core rod 202, a plurality of preform inner cladding capillary tubes 204 of the cladding background material, a plurality of preform stress element rods 206 of the material having the second coefficient of thermal expansion, a plurality of preform outer cladding rods 208 of the cladding background material and a preform stacking tube 210.

The preform stress element rods 206 are arranged in a first stress applying region 422 and in a second stress applying region 424. The preform outer cladding rods 208 are stacked in an outer cladding region 420 around the inner cladding region 212, the first stress applying region 422 and the second stress applying region 424.

Within each stress applying region 422, 424 the preform stress element rods are stacked around a group of preform outer cladding rods 208; the preform outer cladding rods 208 are arranged in three branches of three rods.

Referring to Figure 8, an embodiment provides a polarization maintaining, PM, photonic crystal fibre, PCF, 500 comprising a core region 102 for propagating light in a longitudinal direction of the PM PCF, an inner cladding region 104 surrounding the core region, and an outer cladding region 510 surrounding the inner cladding region.

The outer cladding region 510 comprises cladding background material 112, a first stress applying region 514 and a second stress applying region 516. The cladding background material 112 has the first coefficient of thermal expansion, and may, for example, be Silica.

The first stress applying region and the second stress applying region each extend in the longitudinal direction of the PM PCF through the cladding background material 112 of the outer cladding region 510. The first stress applying region and the second stress applying region are located outside the inner cladding region. The first stress applying region and the second stress applying region are each formed of a plurality of stress elements 1 18, as described above.

The first stress applying region 514 defines a first recess 122 and the second stress applying region 516 defines a second recess 124, as described above.

Within each stress applying region 514, 516, the stress elements are arranged such that each stress element is connected to a number in the range one to five other stress elements. As can be seen in Figure 8, the stress elements 118 in each stress applying region 514, 516 are arranged so that three sections of cladding background material 1 12, comprising two or three lattice points 520, extend into each stress applying region from the outer cladding region 510. Each stress element is therefore connected to no more than five other stress elements.

The locations of the first stress applying region 514 and the second stress applying region 516 relative to the core region 102 and the microstructure elements 106, the first coefficient of thermal expansion, and the second coefficient of thermal expansion are configured to cause a stress induced birefringence in the core region 102 of the PM PCF 500.

Referring to Figure 9, an embodiment provides a preform 600 for fabricating the PM PCF 500 of Figure 8. As described above with reference to Figure 5, the preform 600 comprises stacked longitudinal preform elements comprising a preform core rod 202, a plurality of preform inner cladding capillary tubes 204 of the cladding background material, a plurality of preform stress element rods 206 of the material having the second coefficient of thermal expansion, a plurality of preform outer cladding rods 208 of the cladding background material and a preform stacking tube 210.

The preform stress element rods 206 are arranged in a first stress applying region 622 and in a second stress applying region 624. The preform outer cladding rods 208 are stacked in an outer cladding region 620 around the inner cladding region 212, the first stress applying region 622 and the second stress applying region 624.

Within each stress applying region 622, 624 the preform stress element rods are stacked around a group of preform outer cladding rods 208; the preform outer cladding rods 208 are arranged in three sets of two or three rods extending into the stress applying region from the outer cladding region 620.

Referring to Figure 10, an embodiment provides a polarization maintaining, PM, photonic crystal fibre, PCF, 700 comprising a core region 102 for propagating light in a longitudinal direction of the PM PCF, an inner cladding region 104 surrounding the core region, and an outer cladding region 710 surrounding the inner cladding region.

The outer cladding region 710 comprises cladding background material 112, a first stress applying region 714 and a second stress applying region 716. The cladding background material 112 has the first coefficient of thermal expansion, and may, for example, be Silica.

The first stress applying region and the second stress applying region each extend in the longitudinal direction of the PM PCF through the cladding background material 112 of the outer cladding region 710. The first stress applying region and the second stress applying region are located outside the inner cladding region. The first stress applying region and the second stress applying region are each formed of a plurality of stress elements 1 18, as described above.

The first stress applying region 714 defines a first recess 122 and the second stress applying region 716 defines a second recess 124, as described above.

Within each stress applying region 714, 716, the stress elements are arranged such that each stress element is connected to a number in the range one to five other stress elements. As can be seen in Figure 10, the stress elements 118 in each stress applying region 714, 716 are arranged so that three sections of cladding background material 1 12, comprising two or three lattice points 720, extend into each stress applying region from the outer cladding region 710. Each stress element is therefore connected to no more than five other stress elements.

The locations of the first stress applying region 714 and the second stress applying region 716 relative to the core region 102 and the microstructure elements 106, the first coefficient of thermal expansion, and the second coefficient of thermal expansion are configured to cause a stress induced birefringence in the core region 102 of the PM PCF 700.

Referring to Figure 11 , an embodiment provides a preform 800 for fabricating the PM PCF 700 of Figure 10. As described above with reference to Figure 5, the preform 800 comprises stacked longitudinal preform elements comprising a preform core rod 202, a plurality of preform inner cladding capillary tubes 204 of the cladding background material, a plurality of preform stress element rods 206 of the material having the second coefficient of thermal expansion, a plurality of preform outer cladding rods 208 of the cladding background material and a preform stacking tube 210.

The preform stress element rods 206 are arranged in a first stress applying region 822 and in a second stress applying region 824. The preform outer cladding rods 208 are stacked in an outer cladding region 820 around the inner cladding region 212, the first stress applying region 822 and the second stress applying region 824.

Within each stress applying region 822, 824 the preform stress element rods are stacked around a group of preform outer cladding rods 208; the preform outer cladding rods 208 are arranged in three sets of two or three rods extending into the stress applying region from the outer cladding region 820.

Referring to Figure 12, an embodiment provides a polarization maintaining, PM, photonic crystal fibre, PCF, 900 comprising a core region 102 for propagating light in a longitudinal direction of the PM PCF, an inner cladding region 104 surrounding the core region, and an outer cladding region 910 surrounding the inner cladding region.

The outer cladding region 910 comprises cladding background material 112, a first stress applying region 914 and a second stress applying region 916. The cladding background material 112 has the first coefficient of thermal expansion, and may, for example, be Silica.

The first stress applying region and the second stress applying region each extend in the longitudinal direction of the PM PCF through the cladding background material 112 of the outer cladding region 910. The first stress applying region and the second stress applying region are located outside the inner cladding region. The first stress applying region and the second stress applying region are each formed of a plurality of stress elements 1 18, as described above. The inner cladding region 204 has a substantially hexagonal cross-sectional shape. A first side of the first stress applying region 914 is aligned with a first side of the inner cladding region 204. A first side of the second stress applying region 916 is aligned with a second side of the inner cladding region, opposite the first side of the inner cladding region.

In an alternative embodiment, there may be a gap between each stress applying region and the inner cladding region, which may reduce the birefringence of the PM PCF 900.

Within each stress applying region 914, 916, the stress elements are arranged such that each stress element is connected to a number in the range one to five other stress elements. As can be seen in Figure 12, the stress elements 118 in each stress applying region 914, 916 are arranged in two sets of rows, with a section of cladding background material 112, comprising a row of lattice points 920, between. The outer row (being the row furthest from the core region 102) in each stress applying region also includes two lattice points 920 occupied by cladding background material. Each stress element is therefore connected to no more than five other stress elements.

The locations of the first stress applying region 914 and the second stress applying region 916 relative to the core region 102 and the microstructure elements 106, the first coefficient of thermal expansion, and the second coefficient of thermal expansion are configured to cause a stress induced birefringence in the core region 102 of the PM PCF 900.

Referring to Figure 13, an embodiment provides a preform 1000 for fabricating the PM PCF 900 of Figure 12. The preform comprises stacked longitudinal preform elements comprising a preform core rod 202, a plurality of preform inner cladding capillary tubes 204 of the cladding background material, a plurality of preform stress element rods 206 of the material having the second coefficient of thermal expansion, a plurality of preform outer cladding rods 208 of the cladding background material and a preform stacking tube 210.

The preform inner cladding capillary tubes 204 are stacked in an inner cladding region 212 around the preform core rod. The preform stress element rods 206 are arranged in a first stress applying region 1022 and in a second stress applying region 1024. Within each stress applying region 222, 224 the preform stress element rods are stacked in two sets of rows, with a row of preform outer cladding rods 208 in between. The outer row (being the row furthest from the preform core rod 202) in each stress applying region also includes two preform outer cladding rods 208. Each preform stress element rod is therefore in contact with no more than five other preform stress element rods.

Other arrangements of the preform stress element rods, in which each preform stress element rod is in contact with a number in the range one to five other preform stress element rods, are possible.

The preform outer cladding rods 208 are stacked in an outer cladding region 1020 around the inner cladding region, the first stress applying region and the second stress applying region. The preform core rod, the preform inner cladding capillary tubes, the preform stress element rods and the preform outer cladding rods are located within the preform stacking tube.

The structure of the PM PCFs may enable improved cross-sectional roundness of the PM PCF outer diameter, which may enable improved connectorization of the fibre, in particular when the PM PCF is fixated inside a fibre ferrule e.g., by gluing the fibre to the inner surface of the ferrule. The structure of the stress applying regions may enable improved positioning of the stress applying regions within the PM PCF, which may advantageously provide improved cross- sectional roundness of the PM PCF.

The presence of stress applying regions in the outer cladding region of a PM PCF may result in various distortions of the PM PCF outer surface causing the circumference to deviate from a perfect circle. In some cases, the fibre diameter in the axis of the stress applying regions can differ from the fibre diameter along an axis perpendicular thereto. This can make the cross- sectional shape of the outer surface of the fibre slightly elliptical. In some cases, the introduction of the stress applying regions causes a kink to form on the outer surface of the PM PCF. Such distortions of the outer surface of the fibre can cause problems when the fibre is glued inside a fibre ferrule, where a difference in glue thickness due to the distortion of the fibre circumference may introduce a stress field with counteracts the stress in the fibre core induced by the stress applying regions. In prior art fibres, the distortion of the outer fibre diameter is compensated by increasing the thickness of the outermost fibre region surrounding the outer cladding region to smoothen out the distortion.

One measure of the reduced roundness of the PM PCF caused by the inclusion of the stress applying regions is provided by determining the non-circularity of the PM PCF’s outer surface. The non-circularity describes the deviation of the PM PCF’s cross-sectional shape from a perfect circle and may be determined, based on an approximation of the circumference of an ellipse, as

2 (A-B)/(A+B) where A is the major axis and B is the minor axis of an elliptical fit to the measured radius of the fibre at a plurality of angles from the end face of the fibre.

Alternatively, the non-circularity may be determined as

2B/A where A is the average diameter, A+B is the maximum diameter and A-B is the minimum diameter for a fit of the PM PCF outer diameter, D. for a plurality of circumferential directions, (p, to D((p)=A+Bsin2(p.

In an embodiment, the non-circularity 2 (A-B)/(A+B) is below 0.02, such as below 0.01 , such as below 0.0075, such as below 0.005.

The structure of the stress applying regions of the PM PCFs described above may enable improved positioning of the stress applying regions relative to the inner cladding region and/or the core region, which may advantageously provide improved stress induced birefringence in the core region of the PM PCF. An improved positioning of the stress applying regions may also provide that distortion of the fibre diameter by the stress applying regions is reduced thereby mitigating the associated problems without the need for adding a thick region of fibre material, e.g., a thick ring of cladding background material, surrounding the outer cladding. This provides that a strong stress-induced birefringence can be obtained in a substantially circular fibre without the need for increasing the fibre diameter significantly. For example, in some cases providing that the PM PCF has an outer diameter of 125 pm ensures compatibility with standard fibre optical components while still obtaining a strong birefringence.

The structure of the stress applying regions may enable tilt of the stress applying regions relative to the inner cladding region to be reduced or avoided. Tilt is the angle between any symmetry axis of the inner cladding region and the nearest symmetry axis of the stress applying region.

The structure of the stress applying regions may enable improved shape of the stress applying regions, which may advantageously provide improved cross-sectional roundness of the PM PCF. The improved shape of the stress applying regions may also provide improved stress induced birefringence in the core region of the PM PCF.

Claims

1 . A polarization maintaining, PM, photonic crystal fibre, PCF, comprising: a core region for propagating light in a longitudinal direction of the PM PCF; an inner cladding region surrounding the core region, the inner cladding region comprising a plurality of microstructure elements extending in said longitudinal direction in a cladding background material, said background material having a first coefficient of thermal expansion; and an outer cladding region surrounding the inner cladding region, the outer cladding region comprising a cladding background material having said first coefficient of thermal expansion, a first stress applying region and a second stress applying region, wherein the first stress applying region and the second stress applying region each extend in said longitudinal direction through the cladding background material of the outer cladding region and the first stress applying region and the second stress applying region each comprise a plurality of stress elements of a material having a second coefficient of thermal expansion different to the first coefficient of thermal expansion, wherein the stress elements of each said stress applying region extend in said longitudinal direction and are arranged such that each stress element is connected to a number in the range one to five of other said stress elements, wherein the locations of the first stress applying region and the second stress applying region relative to the core region and the microstructure elements, the first coefficient of thermal expansion, and the second coefficient of thermal expansion are configured to cause a stress induced birefringence in the core region of the PM PCF, wherein the microstructure elements are arranged in a first lattice and wherein the stress elements of each said stress applying region are located at certain lattice points of a respective second lattice such that not every lattice point has a stress element; and wherein the first lattice and the second lattice have the same lattice structure, such as a hexagonal lattice.

2. The PM PCF of claim 1 , wherein the stress elements of each said stress applying region are arranged such that there are no areas of the cladding background material that are enclosed by stress elements.

3. The PM PCF of claim 2, wherein lattice points of the second lattice without stress elements are filled by the cladding background material of the outer cladding region.

4. The PM PCF of claim 3, wherein lattice points of the second lattice without stress elements are not surrounded by stress elements

5. The PM PCF of any one of claims 1 to 4, wherein the first lattice and the second lattice have a hexagonal lattice structure.

6. The PM PCF of any one of claims 1 to 5, wherein the microstructure elements are arranged with a first pitch between centres of neighbouring microstructure elements and stress elements are arranged with a second pitch between centres of neighbouring stress elements.

7. The PM PCF of claim 6, wherein the core region is configured to propagate light at a specified operating wavelength and wherein a ratio of the second pitch to the operating wavelength is greater than 12, such as greater than 13, such as greater than 14, such as greater than 15, such as up to 25.

8. The PM PCF of claim 7, wherein the operating wavelength is lower than 800 nm, such as lower than 750 nm, such as lower than 700 nm, such as lower than 650 nm, such as down to 400nm.

9. The PM PCF of any one of claims 6 to 8, wherein the second pitch is at least 5 |j.m, such as at least 10 |j.m, such as up to 20 |j.m.

11 . The PM PCF of any one of claims 1 to 10, wherein the stress elements comprise rods of a first material having a first refractive index and wherein each stress applying region additionally comprises a second material forming nodes and bridges between the rods, the second material having a second refractive index higher than the first refractive index.

12. The PM PCF of any one of claims 1 to 11 , wherein the inner cladding region comprises at least two layers of microstructure elements surrounding the core region.

13. The PM PCF of any one of claims 1 to 12, wherein the inner cladding region comprises at least 30 microstructure elements arranged in a hexagonal arrangement.

14. The PM PCF of any one of claims 1 to 13, wherein the inner cladding region comprises at least 60 microstructure elements arranged in a hexagonal arrangement.

15. The PM PCF of any one of claims 1 to 14, wherein each of the stress applying regions comprises at least 10 stress elements.

16. The PM PCF of any one of claims 1 to 15, wherein each of the stress applying regions comprises at least 20 stress elements.

17. The PM PCF of any one of claims 1 to 16, wherein each of the stress applying regions comprises at least 28 stress elements.

18. The PM PCF of any one of claims 1 to 17, wherein each of the stress applying regions comprises at least 35 stress elements.

19. The PM PCT of any one of claims 1 to 18, wherein the arrangement of the stress elements in each stress applying region avoids the creation of 7-element clusters of stress elements.

20. The PM PCF of any one of claims 1 to 19, wherein a measure of the non-circularity of the fibre, 2 (A-B)/(A+B), is below 0.02, where A is the major axis and B is the minor axis of an elliptical fit to a measured radius of the fibre at a plurality of angles from an end face of the fibre.

21 . The PM PCF of claim 20, wherein the non-circularity is below 0.01 .

22. A preform for fabricating a PM PCF according to claim 1 , the preform comprising stacked longitudinal preform elements comprising: a preform core rod; a plurality of preform inner cladding capillary tubes of the cladding background material stacked in an inner cladding region around the preform core rod; a plurality of preform stress element rods of said material having said second coefficient of thermal expansion, wherein the preform stress element rods are arranged in a first stress applying region and in a second stress applying region, wherein within each of the first stress applying region and the second stress applying region the preform stress element rods are stacked such that each preform stress element rod is in contact with a number in the range one to five of other said preform stress element rods; a plurality of preform outer cladding rods of the cladding background material stacked in an outer cladding region around the inner cladding region, the first stress region and the second stress region; and a preform stacking tube, wherein the preform core rod, the preform inner cladding capillary tubes, the preform stress element rods and the preform outer cladding rods are located within the preform stacking tube.

23. The preform of claim 22, wherein the preform stress element rods comprise rods of a first material having an outer shell layer of a second material, wherein the first material has a first refractive index and the second material has a second refractive index higher than the first refractive index.

24. The preform of claim 23, wherein the preform stress element rods have an outer diameter, OD, and the outer shell layers have an inner diameter, ID, and wherein a ratio ID/OD of the ID to the OD is in the range 0.8 to 0.95.